Seated or standing, hold the head of a dumbbell with both hands overhead. Keeping the elbows as high as possible, bend the arms to lower the weight as far as you can, then lift into full extension, again trying to keep the elbows high instead of driving them forward.
 
Purpose
The overhead dumbbell tricep extension is a simple exercise for all three heads of the tricep, but emphasizes the long and medial head over the lateral head. Gripping the head of a dumbbell naturally positions the arms well for the exercise.
 
Programming
Generally sets of 6-15 are appropriate.